Question

a= 1/2 + 1/6 +1/12 + 1/20 +1/30 + .... + 1/n. tìm n biết a= 39/40

Answer 1

1/2 + 1/6 + 1/12 + 1/20 + ... + 1/n = 39/40

1/(1x2) + 1/(2x3) + 1/(3x4) + 1/(4x5) + ... + 1/n = 39/40

1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... - 1/n = 39/40

1 -                                                                1/n = 39/40

=> 1/n = 1 - 39/40 = 1/40

=> n = 40